The invention relates to a corrugator unit, particularly for sheets or webs of paper, or similar, of the type comprising at least two rolls having a toothed or corrugated surface, and being mutually engaged and pushed against each other by a predetermined pressure or force.
Said corrugator units are particularly used in corrugated board manufacturing equipment, in which an intermediate corrugated layer is to be interposed between two smooth paper layers.
In prior art corrugator units, the two peripherally corrugated or toothed rolls are supported by end hubs, rotatably about their axis. At least one of the two rolls is compressed against the other by means of pushers, acting on the end supports. Since rolls are considerably long (up to about 2.8 mt.), a uniform compressive force between the corrugator rolls is ensured all over their length by making use of roll deflection. Rolls, usually only one thereof, have a peripheral corrugated surface which is crowned in the axial direction to a predetermined extent, so as to obtain a uniform compression between the two corrugator rolls all along their axial length. The need for an accurate crowned profile of the corrugator rolls forces, especially in case of maximum lengths thereof, to provide very low crowning values, of about 0.2 to 0.6 mm, and therefore to use long-diameter rolls.
Besides the drawbacks related to the considerable mass of the roll, and so to a higher inconvenience in construction, and to higher costs thereof, the above arrangements also provide that flutes are formed on paper, by forcing the latter through the labyrinth formed by the teeth of the rolls, where it is subject to an undesired braking effect, due to friction, which, in some cases, causes paper resistance to be reduced or even paper to be torn.
The necessary crowning values are generally very small, of the order of a few tenths of a millimeter (0.2 to 0.6 mm) and therefore specific pressure values are critically affected by any inaccuracy in construction and by the progressive wear of roll corrugations. Hence, the construction of crowned rolls requires a considerable accuracy, and therefore involves higher manufacturing costs.
Moreover, the profile of corrugator rolls is not homokinetic, being designed according to the intended corrugation, and having variations as regards both the velocity ratio and the distance between centers at each tooth pitch. Owing to this particular construction, combined with the considerable roll masses, vibrations may be generated in the operating condition, which may be subject to important autoamplification phenomena, such as resonance, especially at critical speeds.
The invention has the object to provide a corrugator unit, particularly for sheets or webs of paper, or similar, in such a way that, by means of simple and relatively cheap arrangements, the drawbacks of prior art equipment may be obviated, without jeopardizing the productive efficiency of the corrugator unit and, on the contrary, improving it.
The invention solves the above problem by providing a corrugator unit of the type described hereinbefore, in which the mutual compression of rolls is exerted over the whole axial length thereof.
Particularly, the compressive force of the two rolls against each other is exerted all along the axial length.
The compressive force is exerted uniformly over the whole axial length of the rolls.
With reference to a particular embodiment, one of the two corrugator rolls has a considerably greater diameter, whereas the diameter of the other corrugator roll is considerably smaller than the former, and the mutual compression of said two rolls is exerted on the roll having the smaller diameter.
According to a variant embodiment, the mutual compression of the two rolls is exerted on at least one of the two rolls in at least a few areas or locations of the axial extension of the rolls. Particularly, these locations and the compressive forces associated thereto are distributed along the rolls in such a way as to obtain a uniform compression between the two rolls over the whole axial length thereof.
A first corrugator unit according to the invention has a pair of cooperating and counterrotating corrugator rolls, being supported rotatably about their axis, at their ends. The corrugator roll having the smaller diameter is further held up by a set of supporting belts, rotating on themselves, arranged along the length of the roll, and forming a sort of cradle. Pushers are provided on these belts, for exerting a predetermined force to compress the roll, with the belts, against the second corrugator roll.
Particularly, there may be provided at least one belt-tightening means for each belt, or for groups of supporting belts.
In an alternative embodiment, the compression between the two rolls is obtained through magnetic, particularly electromagnetic means.
A first variant of this embodiment provides an electromagnet having a pair of pole pieces, one pole piece for each roll, so that the magnetic circuit closes on the rolls. In this arrangement, which provides a magnetic gap between the two rolls and between the pole pieces and the two rolls, one or both rolls are pushed or pulled into relative positions, so as to reduce the resistance to the magnetic flux, thereby producing the correct mutual compression of the two rolls.
Generally, the roll having the greater diameter is supported in such a way as to be stationary with respect to movements transverse to the axis of rotation, whereas the other roll, generally the one having the smaller diameter, is rotatably supported and has a sufficient freedom of motion towards the conjunction of roll centers, i.e. in the compression direction.
This variant makes use of the tendency of the magnetic field to close the magnetic gap in order to reach the highest energy condition. In this case, the pole pieces and the magnetic gaps (the one between the two rolls and the one between the roll which is movable in the compression direction and the pole pieces associated thereto) are so dimensioned that the magnetic field generated by one or more coils involves a displacement of said movable roll, such as to generate a specific compressive force against the stationary roll.
A second variant embodiment comprises an electromagnet, whose pole pieces terminate as close as possible to the tangent point between the two rolls. Each pole piece attracts the two rolls by a force having at least one component radial and at least one component substantially circumferential or parallel to the tangent in the contact point between the two rolls. The two circumferential or tangential components, having opposite signs and equal modules, compensate each other by symmetry in the shape of the pole pieces and of their magnetic gaps.
In both cases, the pole pieces, as well as the magnetic field, extend uniformly along the whole length of the rolls.
The above description clearly shows the advantages derived from providing the roll being meant to be pushed with a considerably smaller diameter and mass than the stationary roll.
The advantages of the present invention are selfevident from the above description.
The solution according to the invention allows to overcome all difficulties and drawbacks of prior art corrugator units.
Further, said solution allows to obtain considerable advantages. Particularly, the value of specific pressure, uniformly distributed over the whole length of the roll, may be adapted to real needs, imposed by the type of paper, or similar, being produced.
The diameter of the roll may be chosen as the most appropriate for manufacturing needs, and without accounting for any stiffness requirement thereof.
A roll construction without crowning requires a lower manufacturing accuracy and therefore lower costs.
The corrugator unit according to the invention also allows to avoid any variations in specific pressure, required by roll wear.
By reducing the mass of one roll, resonance frequencies are brought to higher values with respect to manufacturing speeds.
Moreover, the number of paper-gripping teeth or corrugations may be reduced, allowing for the use of a less resistant paper.
The invention also allows to adapt and adjust the distribution of the compressive force in the individual attachment locations, or over the length of the roll, so as to compensate for any variation of the compressive force or as to eliminate any vibration auto-amplification or resonance phenomena, thereby enormously reducing the noise of the corrugator unit, with respect to currently attainable values.
The invention also addresses further improvements, which form the subject of the dependent claims.
The invention is now described in detail, with reference to some embodiments, illustrated in the accompanying drawings, in which: